If you have not been reporting the income and expenses for the "apartment" on schedule E, you should have (and you can amend your returns for the past three years. If you are renting it for an amount well below 'market', your deduction for expenses will be limited to the amount of the rent you report. You can deduct operating expenses including a portion of your insurance, utilities, etc. and any repairs to the basement unit (and a portion of "general" repairs (eg, repairing a broken sewer line). You can depreciate the baement unit share of the cost of your home (excluding the land), and you can depreciate any improvements to the basement unit. You really need to hire a professional to get things set uo properly (most especially if you have not been reporting this in prior years). It will save you a lot more than it costs. You may be able to handle it yourself there after; on the other hand you may find you have found a true friend in your tax man/woman. Suerte! (good luck).